The present invention relates generally to circuitry for controlling automotive ignition systems, and more specifically to circuitry for detecting and terminating ignition coil current.
Modem inductive-type automotive ignition systems typically control the ignition coil such that coil current is allowed to increase to a level high enough to guarantee sufficient spark energy for properly igniting an air/fuel mixture. The inductive nature of an ignition coil dictates that the coil current will increase over time, wherein a control circuit is typically operable to either terminate coil charging after a so-called xe2x80x9cdwell timexe2x80x9d and thereby initiate a spark event, or to dynamically maintain the coil current at a predefined current level for a predefined time period before initiating a spark event. The former technique, commonly referred to as xe2x80x9cramp and firexe2x80x9d, is often preferable over the latter technique, commonly known as xe2x80x9cramp and holdxe2x80x9d, in that closed-loop stability is typically not an issue for concern in a ramp and fire system. Moreover, power dissipation in a coil current switching device is substantially reduced in a ramp and fire system since the switching device is only required to operate in a xe2x80x9csaturatedxe2x80x9d mode with low voltage across its terminals. By contrast, a ramp and hold system requires linearly controlling the coil current such that the coil current becomes limited by the resistance of the ignition coils and the voltage across it. This requires increasing the voltage drop across the coil current switching device which then corresponds to a proportional increase in switching device power dissipation.
One known example of a xe2x80x9cramp and firexe2x80x9d ignition system 10 of the type just described is illustrated in FIG. 1, wherein system 10 includes an ignition control circuit 12 having an electronic spark timing (EST) buffer circuit 14 receiving an EST control signal from a control circuit 16 via signal path 18. The EST buffer circuit 14 buffers the EST control signal and provides a buffered EST control signal ESTB to a gate drive circuit 20. The gate drive circuit 20 is responsive to the ESTB signal to supply a gate drive signal GD to a gate 22 of an insulated gate bipolar (IGBT) transistor 24 or other coil switching device via signal path 26. A collector 28 of IGBT 24 is connected to one end of a primary coil 30 forming part of an automotive ignition coil having an opposite end connected to battery voltage VBATT. An emitter 32 of IGBT 24 is connected to one end of a sense resistor RS having an opposite end connected to ground potential, and to a non-inverting input of a comparator 36 via signal path 38. An inverting input of comparator 36 is connected to a reference voltage VREF, and an output of comparator 36 supplies a trip voltage VTRIP to gate drive circuit 20.
In the operation of system 10, gate drive circuit 20 is responsive to a rising edge of an ESTB signal to supply a full gate drive signal GD to the gate 26 of IGBT 24. As IGBT 24 begins to conduct in response to the gate drive signal GD, a coil current IC begins to flow through primary coil 30, through IGBT 24 and through RS to ground, thereby establishing a xe2x80x9csense voltagexe2x80x9d VS across resistor RS. As the coil current IC increases due to the inductive nature of coil primary 30, the sense voltage VS across RS likewise increases until it reaches the comparator reference voltage VREF. At this point, the comparator 36 switches state and the corresponding change in state of the trip voltage VTRIP causes the gate drive circuit 20 to turn off or deactivate the gate drive voltage GD so as to inhibit the flow of coil current IC through the primary coil 30 and coil current switching device 24. This interruption in the flow of coil current IC through primary coil 30 causes primary coil 30 to induce a current in a secondary coil coupled thereto (not shown), wherein the secondary coil is responsive to this induced current to generate an arc across the electrodes of a spark plug connected thereto (not shown in FIG. 1).
One drawback to a ramp and fire ignition system of the type illustrated in FIG. 1 is that under low vehicle battery voltage (VBATT) conditions, the resistance of the primary ignition coil 30 may limit the ability to achieve maximum coil current IC. The resistance of primary coil 30 is typically a function of the physical construction of the coil 30, and is also a function of temperature with the resistance of coil 30 increasing as temperature increases. Under certain high temperature and low battery voltage operating conditions, the coil current IC therefore may not be able to increase to the level at which the corresponding sense voltage VS reaches the comparator reference voltage VREF. In operation under such conditions, the coil current IC may thus increase only to its resistively limited level with VS less than VREF, and remain at that level until some other control mechanism terminates the current ignition dwell event. For example, in some known ignition systems, such backup control is effectuated by a so-called xe2x80x9cover-dwellxe2x80x9d or xe2x80x9cdwell timeoutxe2x80x9d timing circuit that commands the coil current switching device (e.g., IGBT 24) to turn off after some predetermined time period. However, in some ignition systems, such a dwell time extension may not be an acceptable strategy for addressing low coil current conditions that result in VS less than VREF.
What is therefore needed is an improved automotive ignition control strategy that addresses the foregoing drawbacks of known automotive ignition control systems.
The foregoing shortcomings of the prior art are addressed by the present invention. In accordance with one aspect of the present invention, an ignition control circuit comprises a comparator circuit defining a first input receiving a variable input signal, a second input and an output producing a trip signal, a first circuit producing a first current as a function of temperature, and a second circuit producing a second current, wherein the second current is a function of battery voltage below a predefined battery voltage threshold and otherwise zero, and wherein the first and second currents combine at the second input of the comparator circuit to define a reference level at which the trip signal changes state in response to the variable input signal.
In accordance with another aspect of the present invention, an ignition control circuit comprises a comparator circuit defining a first input receiving a variable input voltage, a second input and an output producing a trip signal, a first circuit supplying a reference voltage to the second input of the comparator, wherein the reference voltage is a function of temperature and of battery voltage and defines a reference level at which the trip signal changes state, and a second circuit responsive to a control signal to reduce the reference voltage to a predefined fraction thereof.
In accordance with a further aspect of the present invention, a method of producing a reference voltage for an ignition control circuit comprises the steps of establishing a first current as a function of temperature, establishing a second current, wherein the second current is a function of battery voltage below a battery voltage threshold and otherwise zero, combining the first and second currents and producing a reference voltage therefrom, and comparing a variable input voltage with the reference voltage and producing a trip signal based thereon.
One object of the present invention is to provide an improved automotive ignition control system by implementing an ignition control circuit defining a coil current trip level reference as a function of temperature and battery voltage.
Another object of the present invention is to provide such a circuit further defining the coil current trip level reference as a function of engine speed.
These and other objects of the present invention will become more apparent from the following description of the preferred embodiments.